Modern drilling techniques employ an increasing number of sensors in downhole tools to determine downhole conditions and parameters such as pressure, spatial orientation, temperature, gamma ray count etc. that are encountered during drilling. These sensors are usually employed in a process called ‘measurement while drilling’ (MWD). The data from such sensors are either transferred to a telemetry device, and thence up-hole to the surface, or are recorded in a memory device by ‘logging’.
One type of telemetry method is electromagnetic (EM) telemetry, which uses a downhole EM transmitter to create very low frequency EM carrier signal waves in the formation adjacent to the well that are detected at the surface. In EM telemetry systems, the downhole carrier signal is produced by applying an alternating electric current across an electrically isolated (nonconductive) portion of the drill string. The required isolation is provided by a mechanically strong gap in a portion of drill string (called a ‘sub’) in order to maintain the torsional, bending etc. properties required for the drilling process. The EM signal originating across the gap is subsequently detected on the surface by, in general, measuring the induced electric potential difference between the drill rig and a grounding rod located in the earth some distance away.
Current measurements include resistivity measurements taken by a MWD tool sub that has an insulated gap joint. Because of the lengthy electrically conductive drill collars above and below the gap joint, the vertical resolution is reduced. While the measurements from the MWD tool sub can be used to interpret conditions at the drill bit, the remote location of the MWD tool sub from the drill bit will affect the accuracy of bit resistivity and other measurements.